Mercury accumulation in raptors

Abstract

Mercury (Hg) is a toxic heavy metal that when methylated to form methylmercury (MeHg), bioaccumulates in exposed organisms and biomagnifies through food webs. Most studies examining Hg concentrations in birds of prey have focused on species associated with aquatic systems such as Bald Eagle (Haliaeetus leucocephalus). My goal was to assess Hg concentrations in multiple species of migrating raptors in the upper Midwestern US. From 2009-2012, 966 raptors of 11 species were captured at Hawk Ridge, Duluth, MN, USA. Breast feathers were sampled and analyzed for total Hg concentrations, which is a good analog for methlymercury. Mean Hg concentrations ranged from 0.11 – 3.46 ppm. Carbon and nitrogen stable isotope values were analyzed for Sharp-shinned Hawk (Accipiter striatus) and Merlin (Falco columbarius) because they had the highest mean Hg concentrations with 3.46 and 2.15 ppm respectively. Stable isotope analysis suggested that both species consumed terrestrial prey and that total Hg concentration increased with trophic level. Further analysis of Sharp-shinned Hawk and Merlin feather samples show Hg increases with age. The Hg concentrations observed in Sharp-shinned Hawks, Merlins, and Northern Goshawks (Accipiter gentilis) represent a concern when compared with concentrations found in the American Kestrel (Falco sparverius); one of the few raptors with enough experimental data for comparison.